Cause analysis and treatment of local overheating

Cause analysis and treatment of local overheating of enclosed bus shell

0 preface

the aluminum shell of fully connected isolated phase enclosed bus can play a good role in sealing and isolating the bus, basically eliminating the grounding and interphase short circuit accidents caused by external moisture, dust and other foreign objects, and ensuring personal safety and high reliability. The circulating current generated by the electromagnetic induction of the fully connected shell circuit (the value is approximately equal to the bus current, but the direction is opposite) can make most of the magnetic field outside the shell disappear, thus eliminating the induced loss and heating of the steel structure outside the shell. Due to its advantages, the fully connected isolated phase enclosed bus is widely used in generator outgoing lines and auxiliary branch lines above 8000A, but its heating problem should also be paid attention to in use

1 question raising

1 1. Defects found

in late April 2003, the electrical operation attendant of Huaibei Power Plant, during the patrol inspection of the equipment when the No. 5 generator (which has been upgraded to 220MW) is running with 200MW load, measured its outgoing phase B (middle phase) closed bus with far-infrared thermometer, and the detachable extension force acting on the outer flange of the span center contraction flexible junction box is locally overheated, The temperature is as high as 125 ℃ (the temperature in other parts of the flange is also higher, and significantly higher than the relative position of the edge phase). After the peak load, reduce the unit output, and the temperature in this area will decrease, but the temperature at the overheat is still not less than 100 ℃ at 150MW load, and the temperature in other parts of the flange is also about 70 ℃, exceeding the ministerial regulations. If it is overheated for a long time, it will cause rapid aging of the rubber expansion joint and damage, thus losing the sealing performance of the enclosed bus

1.2 cause analysis

(1) data show that due to the skin effect and proximity effect of the circulation of the fully connected enclosed bus shell, the ratio of the middle phase additional resistance of the three-phase side-by-side circular tube current carrying conductors to the three-phase average additional resistance is close to twice, so the temperature rise of the middle phase enclosed bus shell is higher than that of the side phase

(2) there are 12 pieces of 120mm cross-section in the expansion joint of the enclosed bus of No. 5 generator in Huaibei Power Plant × 10mm jumper aluminum bar (evenly distributed), in view of the skin effect loss coefficient of circular conductor kf=r/r0 ≈ 1 (where: R is AC resistance, R0 is DC resistance), it can be considered that the circulating current generated by the enclosed bus shell circuit due to electromagnetic induction is also evenly distributed, that is, under normal circumstances, the circulating current passed by each jumper aluminum bar is about 1/12 of the load current of the generator

(3) although the jumper aluminum bar at the expansion joint is built-in, it can be preliminarily judged that the built-in jumper aluminum bar at the overheat has poor contact. According to the calculation formula of circulating current loss of fully connected enclosed bus shell, pc=i2krkokf Where IK is shell circulation; RKO is the DC resistance of the housing; KF is the skin effect loss coefficient of the shell, which can be taken as 1 when the thickness is not greater than 8mm

it is obvious that with the poor contact of the jumper aluminum bar hidden in the overheat, the DC resistance RKO of this part will rise synchronously, causing the circulation loss PC to rise in a positive proportion, which is bound to cause local overheating. And with the change of unit load, the shell circulation IK changes accordingly, and the circulation loss PC is proportional to the square of the shell circulation (effective value), and the temperature rise must change accordingly

2 treatment method

2.1 similar determination of the treatment scheme in operation

(1) before the defects are treated, contact the provincial middle note to appropriately reduce the active and reactive loads of generator 5. The electrical operation attendant should strengthen patrol inspection, do a good job in continuous temperature measurement, install a temporary exhaust fan, blow air outside to reduce the temperature, and try to control the temperature at the overheat

(2) there are 24 fixing bolts on each side of the flange outside the flexible connection box at both ends of the expansion joint. It is planned to install connecting small aluminum bars between each pair of bolts one by one, in order to implement diversion of the built-in jumper aluminum bars, so as to reduce heating

since the enclosed bus shell is three-phase short circuited and grounded, there is no potential difference between the axial electromotive force induced on the aluminum shell and the earth, so it can be implemented without shutdown

2.2 treatment process after shutdown

since the unit was shut down for standby, it was treated after shutdown

(1) disassemble the rubber sleeve of the expansion joint. Although the fixing bolts of each jumper aluminum bar are not seriously loose after inspection. However, some fastening screws have obvious electrochemical corrosion, resulting in poor contact and overheating traces, especially in overheated places, especially for fixing screws that should be used as flame retardants and high-molecular synthetic material boosters. Untie the 12 bridging transition aluminum bars, polish the overheated and electrochemical corrosion parts, and then apply power grease with enhanced conductivity and oxidation resistance to eliminate the heating caused by large contact resistance

(2) considering that No. 5 generator unit has been in operation for many years (put into operation in 1981), in order to enhance the carrying capacity at the connection of phase B enclosed bus expansion joint, 24 pieces with a section of 50mm are added between the outer flanges of the flexible connection box at both ends of phase B expansion joint according to the above treatment scheme × 5mm small aluminum bars are connected in order to shunt the 12 built-in jumper aluminum bars and increase the surface heat dissipation capacity. The treatment is shown in Figure 1

3 effect detection

(1) at the beginning of May 2003, when the No. 5 generator set was running with 200MW load, the temperature at the original overheating of phase B of the enclosed bus was measured by far-infrared thermometer as 50 ℃, and the temperature at other parts of the flange also fell below 50 ℃. When the high temperature weather in summer is 210MW, the temperature at the flange of the expansion joint of phase B enclosed bus is also below 65 ℃, and the cooling effect is obvious. No. 5 generator unit successfully completed the task of peak summer

(2) when the load is 210MW, use a clamp on ammeter to measure the newly installed B? The current of each small aluminum bar connected at the expansion joint of phase enclosed bus is about 25A, which achieves the expected purpose of shunting

conclusion

through the discovery, analysis and treatment of local overheating of the outer flange of the flexible joint box of the closed bus of 200MW generator set on the basis of the conditions and basic conditions for sustainable development, it is suggested that the power plant with large units should pay attention to the following matters:

(1) it is very necessary and important for the far-infrared thermometer to be used in the inspection of the operation site of the electrical profession

(2) the expansion joint of the enclosed bus of the generator should be included in the inspection equipment of the electrical discipline, and the heating should be taken as an important inspection content

(3) once the overheating phenomenon is found at the outer flange of the expansion joint, when there is no chance of shutdown in a short time, the treatment scheme in operation can be adopted to reduce the unplanned shutdown of the unit

(4) during unit maintenance, carefully check and repair the interface of jumper aluminum bar hidden in the expansion joint according to the specific situation of temperature rise in the operation of generator enclosed bus, so as to fully ensure the availability of generator unit. (Yang yonghuai)